Light emitting device

ABSTRACT

A light emitting device according to one embodiment includes a board; a light emitting element mounted on the board, emitting light having a wavelength of 250 nm to 500 nm; a red fluorescent layer formed on the element, including a red phosphor expressed by equation (1), having a semicircular shape with a diameter r; 
       (M 1−x1 Eu x1 ) a Si b AlO c N d   (1)
 
     (In the equation (1), M is an element that is selected from IA group elements, IIA group elements, IIIA group elements, IIIB group elements except Al (Aliminum), rare-earth elements, and IVB group elements), 
     an intermediate layer formed on the red fluorescent layer, being made of transparent resin, having a semicircular shape with a diameter D; and a green fluorescent layer formed on the intermediate layer, including a green phosphor, having a semicircular shape. A relationship between the diameter r and the diameter D satisfies equation (2): 
       2.0 r  (μm)≧ D ( r +1000) (μm).  (2)

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority fromJapanese Patent Application No. 2010-199983, filed on Sep. 7, 2010; theentire contents of which are incorporated herein by reference.

FIELD

Embodiments described herein relate generally to a light emittingdevice.

BACKGROUND

Recently, attention focuses on a so-called white-color Light EmittingDevice (LED) in which a yellow phosphor such as YAG:Ce is combined witha blue LED to emit white-color light by single chip. Conventionally, theLED emits red, green, or blue light in monochromatic form, and it isnecessary that plural LEDs emitting monochrome wavelengths are driven inorder to emit the white-color light or intermediate-color light.However, currently, the combination of the light emitting diode and thephosphor removes the trouble to obtain the white-color light with asimple structure.

An LED lamp in which the light emitting diode is used is applied tovarious display devices of a mobile device, a PC peripheral device, anOA device, various switches, a light source for backlight, and a displayboard. In the LED lamps, there is a strong demand for high efficiency.Additionally, there is a demand for high color rendering ingeneral-purpose lighting applications, and there is a demand for highcolor gamut in LCD TV backlight applications. High efficiency of thephosphor is required for the purpose of the high efficiency of the LEDlamp, and a white-color light source in which a phosphor emitting blueexcitation light, a phosphor excited by blue light to emit green light,and a phosphor excited by blue light to emit red light are combined ispreferable to the high color rendering and the high color gamut.

The high-power LED generates heat by drive, and generally the phosphoris heated up to about 100 to about 200° C. When the temperature rise isgenerated, generally emission intensity of the phosphor is degraded togenerate so-called thermal quenching. Therefore, unfortunately theluminous efficiency is degraded particularly in a high-temperaturerange, that is, a high-current range.

Additionally, when plural phosphors are used, unfortunately the luminousefficiency is degraded by reabsorption between phosphors.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view illustrating a light emittingdevice according to a first embodiment;

FIG. 2 is a view illustrating an absorptance of a red phosphor of thefirst embodiment;

FIG. 3 is a view illustrating function of the light emitting device ofthe first embodiment;

FIG. 4 is a view illustrating a luminous flux loss of the light emittingdevice of the first embodiment;

FIG. 5 is a schematic sectional view illustrating a light emittingdevice according to a second embodiment; and

FIG. 6 is a wiring diagram of a white-color light emitting module ofExamples.

DETAILED DESCRIPTION

A light emitting device according to one embodiment includes a board; alight emitting element mounted on a principal surface of the board, thelight emitting element emitting light having a wavelength of 250 nm to500 nm; a red fluorescent layer formed on the light emitting element,the red fluorescent layer including a red phosphor expressed by equation(1), an outer circumference of the red fluorescent layer having asemicircular shape with a diameter r in a section perpendicular to theprincipal surface;

(M_(1−x1)Eu_(x1))_(a)Si_(b)AlO_(c)N_(d)  (1)

(In the equation (1), M is an element that is selected from IA groupelements, IIA group elements, IIIA group elements, IIIB group elementsexcept Al (Aliminum), rare-earth elements, and IVB group elements, andx1, a, b, c, and d satisfy the following relationship:

0<x1≦1,

0.60<a<0.95,

2.0<b<3.9,

0.04≦c≦0.6,

4<d<5.7)

an intermediate layer formed on the red fluorescent layer, theintermediate layer being made of transparent resin, an outercircumference of the intermediate layer having a semicircular shape witha diameter D in a section perpendicular to the principal surface; and agreen fluorescent layer formed on the intermediate layer, the greenfluorescent layer including a green phosphor, an outer circumference ofthe green fluorescent layer having a semicircular shape in a sectionperpendicular to the principal surface. A relationship between thediameter r and the diameter D satisfies equation (2):

2.0r (μm)≦D≦(r+1000) (μm).  (2)

Embodiments will be described below with reference to the drawings.

As used herein, the red phosphor means a phosphor that emits lightranging from an orange color to a red color (hereinafter alsocollectively referred to as red color), that is, light having a peak atthe wavelength of 580 to 700 nm, which is longer than the excitationlight, when the phosphor is excited by the light having the wavelengthof 250 nm to 500 nm, that is, the near-ultraviolet light or the bluelight.

As used herein, the green phosphor means a phosphor that emits lightranging from a blue-green color to a yellow-green color (hereinafteralso collectively referred to as green color), that is, light having apeak at the wavelength of 490 to 550 nm, which is longer than theexcitation light, when the phosphor is excited by light having thewavelength of 250 nm to 500 nm, that is, the near-ultraviolet light orthe blue light.

As used herein, the “white-color light” means a concept including alightbulb color, a warm white color, a white color, a day white light, and aday light color, in which pieces of light having different wavelengthsused generally in the lighting device are mixed.

First Embodiment

A light emitting device according to a first embodiment includes theboard that includes the principal surface on which the light emittingelement is mounted; the light emitting element that is mounted on theprincipal surface to emit the light having the wavelength of 250 nm to500 nm; the red fluorescent layer that is formed on the light emittingelement to include the red phosphor expressed by the equation (1), theouter circumference being formed into the semicircular shape having thediameter r in the section perpendicular to the principal surface;

(M_(1−x1)Eu_(x1))_(a)Si_(b)AlO_(c)N_(d)  (1)

(In the equation (1), M is an element that is selected from IA groupelements, IIA group elements, IIIA group elements, IIIB group elementsexcept Al(Aliminum), rare-earth elements, and IVB group elements. x1, a,b, c, and d satisfy the following relationship.

0<x≦1,

0.60<a<0.95,

2.0<b<3.9,

0.04≦c≦0.6,

4<d<5.7)

the transparent resin intermediate layer that is formed on the redfluorescent layer, the outer circumference being formed into thesemicircular shape having the diameter D in the section perpendicular tothe principal surface; and the green fluorescent layer that is formed onthe intermediate layer to include the green phosphor, the outercircumference being formed into the semicircular shape in the sectionperpendicular to the principal surface. A relationship between thediameter r and the diameter D satisfies equation (2).

2.0r (μm)≦D≦(r+1000) (μm)  (2)

The sialon phosphor having the composition expressed by the equation (1)is a red phosphor (R). The red phosphor (R) emits the light ranging fromthe orange color to the red color, that is, the light having the peak atthe wavelength of 580 to 700 nm, which is longer than the excitationlight, when the red phosphor (R) is excited by the light having thewavelength of 250 nm to 500 nm, that is, the near-ultraviolet light orthe blue light.

Because of the small thermal quenching, the red phosphor obtains theexcellent luminous efficiency even in a high temperature region. At thesame time, an excitation spectrum of the red phosphor becomes extensivefrom the near-ultraviolet light to the green light. Therefore, when thewhite-color light emitting device (white-color LED) is formed by acombination of the red phosphor and the green phosphor, the red phosphorsignificantly reabsorbs the green light to possibly generate thedegradation of the luminous efficiency or the color shift.

In the light emitting device of the first embodiment, the transparentresin intermediate layer is provided between the red fluorescent layerand the green fluorescent layer, and the diameters of the redfluorescent layer and intermediate layer are restricted. Therefore, thereabsorption of the green light by the sialon red phosphor is suppressedto implement the white-color light emitting device, in which theluminous efficiency is increased and the color shift is suppressed.

FIG. 1 is a schematic sectional view illustrating the light emittingdevice of the first embodiment. A light emitting device 10 is awhite-color LED that emits the white-color light. The light emittingdevice 10 includes a board 12 that includes the principal surface onwhich the light emitting element is mounted. For example, the board 12is made of a highly-reflective material. The principal surface means aplane in an upper surface of the board.

For example, the blue LED chip that is a light emitting element 14 ismounted on the principal surface of the board 12, and the light emittingelement 14 emits the light having the wavelength of 250 nm to 500 nm.For example, the blue LED chip is connected to wiring (not illustrated)through a gold wire 16. Driving currents are supplied to the blue LEDchip from the outside through the wiring, whereby the blue LED chipemits the blue light for excitation.

A hemispherical element sealing transparent layer 18 made of atransparent resin is provided on the light emitting element 14. Forexample, a silicone resin is used as the transparent resin.

A red fluorescent layer 20 is formed such that the element sealingtransparent layer 18 is covered therewith. The outer circumference ofthe red fluorescent layer 20 is formed into the semicircular shapehaving the diameter r in the section perpendicular to the principalsurface. The red fluorescent layer 20 includes the red phosphor having acomposition expressed by the equation (1).

(M_(1−x1)Eu_(x1))_(a)Si_(b)AlO_(c)N_(d)  (1)

(In the equation (1), M is an element that is selected from IA groupelements, IIA group elements, IIIA group elements, IIIB group elementsexcept Al, rare-earth elements, and IVB group elements. x1, a, b, c, andd satisfy the following relationship.

0<x1≦1,

0.60<a<0.95,

2.0<b<3.9,

0.04≦c≦0.6,

4<d<5.7)

Desirably, the element M is strontium (Sr).

For example, the red fluorescent layer 20 is formed while the redphosphor is dispersed in a transparent silicone resin. The redfluorescent layer 20 absorbs the blue light emitted from the blue LEDand converts the blue light into the red light.

A transparent resin intermediate layer 22 is formed on the redfluorescent layer 20. The outer circumference of the intermediate layer22 is formed into the semicircular shape having the diameter D in thesection perpendicular to the principal surface of the board 12. Forexample, a silicone resin is used as the transparent resin.

A green fluorescent layer 24 including the green phosphor is formed suchthat the intermediate layer 22 is covered therewith, and the outercircumference of the green fluorescent layer 24 is formed into thesemicircular shape in the section perpendicular to the principalsurface. The reabsorption by the red fluorescent layer 20 is suppressedby providing the intermediate layer 22.

For example, the green fluorescent layer 24 is formed while the greenphosphor is dispersed in the transparent silicone resin. The greenfluorescent layer 24 absorbs the blue light emitted from the blue LEDand converts the blue light into the green light.

An outer surface transparent layer (outer surface layer) 26 made of, forexample, the transparent silicone resin, is formed such that the greenfluorescent layer 24 is covered therewith. The green fluorescent layer26 has a function of suppressing total reflection of the pieces oflight, which are emitted from the light emitting element 14, the redfluorescent layer 20, and the green fluorescent layer 24, at aninterface with an atmosphere.

Thus, the light emitting device 10 includes the red fluorescent layer20, intermediate transparent layer 22 made of transparent resin, andgreen fluorescent layer 24, which are stacked into the hemisphericalshape on the light emitting element 14. The light emitting device 10emits the white-color light having the high emission intensity and highcoloring homogeneity by forming the fluorescent layers into thehemispherical shape.

The relationship between the diameter r outside the red fluorescentlayer 20 and the diameter outside the intermediate layer 22 satisfiesthe following equation (2).

2.0r (μm)≦D≦(r+1000) (μm)  (2)

The red fluorescent layer 20 and the intermediate layer 22 are permittedto deviate from the perfect hemispherical shape due to factors on theproduction, for example. In such cases, the diameter r or the diameter Dmay be computed by averaging a diameter in a direction perpendicular tothe principal surface of the board 12 and a diameter in a directionparallel to the principal surface of the board 12.

Next, function of the light emitting device 10 will be described.

FIG. 2 is a view illustrating normalized emission intensity of thesialon red phosphor having the composition expressed by the equation(1). In FIG. 2, a horizontal axis indicates a wavelength of excitationlight, and a vertical axis indicates the normalized emission intensityat a monitoring wavelength of 602 nm.

An absorptance of 87% at the wavelength of 457 nm is determined from anemission property evaluation when the excitation is performed by theblue LED having the peak wavelength of 457 nm. The absorptance of 68% ofthe green light at the wavelength of 525 nm is determined by aproportional distribution from the absorptance of 87% and the propertyof FIG. 2. Hereinafter an absorptance β of the green light by the redphosphor of the first embodiment is representatively discussed by thevalue of 0.68.

Practically a luminous flux loss caused by the reabsorption of the greenlight by the red fluorescent layer 20 is desirably set to 5% or lessfrom the viewpoint of the property of the light emitting device. Therelationship required for the diameter r outside the red fluorescentlayer 20 and the diameter D outside the intermediate layer 22 will bederived in order to satisfy the condition that the luminous flux loss is5% or less.

FIG. 3 is an explanatory view of action of the light emitting device ofthe first embodiment. For example, the green light emitted from aposition A in the green fluorescent layer 24 spread all around asillustrated in FIG. 3. In FIG. 2, a luminous flux in a rangeconceptually indicated by two arrows and a dotted-line ellipsoid, thatis, a luminous flux in a range where the red fluorescent layer is seenfrom the position A becomes a luminous flux that is possibly absorbed bythe red fluorescent layer 20.

A ratio L₂/L₁ of a luminous flux L₂ (the luminous flux that is possiblyabsorbed by the red fluorescent layer 20) reaching the outer surface ofthe red fluorescent layer 20 to a total luminous flux L₁ from the greenfluorescent layer 24 is expressed by the following equation (4).

L ₂ /L ₁=2π(1−(r/D)²)^(−1/2))/2π=1−(1−(r/D)²)^(−1/2))  (4)

A luminous flux loss γ generated by the reabsorption of the green lightby the red fluorescent layer 20 is expressed by a product of theabsorptance β (=0.68) of the green light by the red phosphor, the rationL₂/L₁, and a luminous factor difference δ (=0.63) between the greenlight (wavelength of 525 nm) and the red light (wavelength of 600 nm).That is, the luminous flux loss γ is expressed by the following equation(5).

γ=βδ(1−(1−(r/D)²)^(−1/2)))  (5)

FIG. 4 is a graph illustrating the luminous flux loss of the lightemitting device of the first embodiment. In FIG. 4, the horizontal axisindicates D/r, and the vertical axis indicates the luminous flux loss γcomputed from the equation (4). At this pint, computation is performedusing β=0.68 and δ=0.63.

As is clear from FIG. 4, in order to obtain the luminous flux loss γ of5% or loss, desirably D/r is not lower than 2.0 and more desirably D/ris not lower than 2.2. Therefore, desirably the following equation (6)is satisfied. When D/r is not lower than 2.0, a tendency to saturate theluminous flux loss becomes preferably prominent.

2.0r (μm)≦D (μm)  (6)

However, when the intermediate layer 22 is excessively thickened, theluminous flux loss caused by the absorption by the intermediate layer 22becomes obvious to possibly lose the effect that the diameters of thered fluorescent layer 20 and intermediate layer 22 are restricted. Thesilicone resin that is the typical transparent resin has a transmittanceof 90% in a range of the near-ultraviolet light to the blue light withrespect to a resin plate having a thickness of 2000 μm. That is, theabsorptance is 10%.

It is necessary that a the thickness (D−r) of the intermediate layer 22satisfy the following equation (7) in order to suppress the luminousflux loss, caused by the absorption of the excitation light or thephosphor emission in the intermediate layer 22, to 5% or less.

(D−r)≦1000 (μm)  (7)

In order to suppress the effect of the intermediate layer 22, desirablythe thickness (D−r) of the intermediate layer 22 is 500 μm or less, moredesirably the thickness (D−r) is 200 μm or less.

Accordingly, in the light emitting device, it is necessary that thediameter r outside the red fluorescent layer 20 and the diameter Doutside the intermediate layer 22 satisfies the following equation (2)from the equation (6) and the equation (7) in order that the absorptionlosses by the red fluorescent layer 20 and the intermediate layer 22 issuppress to the practically-required value of 5% or less.

2.0r (μm)≦D≦(r+1000) (Ξm)  (2)

Therefore, the first embodiment implements the light emitting device inwhich the reabsorption of the green light by the red fluorescent layeris suppressed exert the excellent luminous efficiency while the redphosphor having the small thermal quenching is used.

In the light emitting device of the first embodiment, desirably thegreen phosphor has the composition expressed by the following equation(3).

(M′_(1−x2)Eu_(x2))_(3−y)Si_(13−z)Al_(3+z)O_(2+u)N_(21−w)  (3)

(In the equation (3), M′ is an element that is selected from IA groupelements, IIA group elements, IIIA group elements, IIIB group elementsexcept Al, rare-earth elements, and IVB group elements. x2, y, z, u, andw satisfy the following relationship.

0<x2≦1,

−0.1≦y≦0.15,

−1≦z≦1,

−1<u−w≦1.5)

The sialon phosphor having the composition expressed by the equation (3)is a green phosphor (G). The green phosphor (G) emits the light rangingfrom the blue-green color to the yellow-green color, that is, the lighthaving the peak at the wavelength of 490 to 580 nm, which is longer thanthe excitation light, when the green phosphor (G) is excited by thelight having the wavelength of 250 nm to 500 nm, that is, thenear-ultraviolet light or the blue light.

The green phosphor (G) has the small thermal quenching to realize theexcellent luminous efficiency particularly in a high temperature region.Accordingly the high-efficiency light emitting device having the betterthermal quenching property can be implemented.

Desirably, the element M′ is strontium (Sr).

Second Embodiment

The blue LED is used in the light emitting element of the firstembodiment. On the other hand, a near-ultraviolet LED is used in a lightemitting element according to a second embodiment, and the lightemitting device of the second embodiment includes a blue fluorescentlayer. Therefore, the descriptions of the contents overlapped with thoseof the first embodiment are omitted.

FIG. 5 is a schematic sectional view illustrating the light emittingdevice of the second embodiment. A light emitting device 20 is awhite-color LED that emits the white-color light. In the light emittingdevice 20, the light emitting element 14 is the near-ultraviolet LEDthat emits near-ultraviolet light. A blue fluorescent layer 28 isfurther formed between the green fluorescent layer 24 and the outersurface transparent layer 26.

For example, the blue fluorescent layer 28 is formed while a bluephosphor is dispersed in a transparency silicone resin. The bluefluorescent layer 28 absorbs the near-ultraviolet light emitted from thenear-ultraviolet LED and converts the near-ultraviolet light into bluelight.

Other configurations of the second embodiment are similar to those ofthe first embodiment. In the light emitting device of the secondembodiment, similar to the first embodiment, the reabsorption of thegreen light by the red fluorescent layer is suppressed exert theexcellent luminous efficiency while the red phosphor having the smallthermal quenching is used.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the light emitting device describedherein may be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the devices andmethods described herein may be made without departing from the spiritof the inventions. The accompanying claims and their equivalents areintended to cover such forms or modifications as would fall within thescope and spirit of the inventions.

For example, a semiconductor light emitting element that emits thenear-ultraviolet light or the blue light may be used as the lightemitting element that emits the excitation light used in the lightemitting device. For example, a gallium nitride compound semiconductorcan be used as the LED.

In the embodiments, the intermediate layer is directly formed on the redfluorescent layer by way of example. Alternatively, for example, ayellow fluorescent layer including a yellow phosphor may be formedbetween the red fluorescent layer and the intermediate layer.

It is desirable to form the element sealing transparent layer and theouter surface transparent layer. However, it is not always necessary toform the element sealing transparent layer and the outer surfacetransparent layer. One of or both the element sealing transparent layerand the outer surface transparent layer may be omitted.

Any binder resin can be used as the binder resin that constitutes thebase material of the sealing resin as long as the binder resin issubstantially transparent in the neighborhood of the peak wavelength ofthe light emitting element (excitation element) and the wavelength rangelonger than the peak wavelength of the light emitting element. Generallyexamples of the binder resin include a silicone resin, an epoxy resin, apolydimethylcyclohexan derivative having an epoxy group, an oxetaneresin, an acrylic resin, a cycloolefin resin, a urea resin, a fluorineresin, and a polyimide resin.

EXAMPLES Examples 1 to 5

FIG. 6 is a wiring diagram of a white-color light emitting module ofExamples. The light emitting devices 10 of the first embodimentillustrated in FIG. 1 were connected so as to become a 4-by-4 arrayillustrated in FIG. 6, and an anode electrode 60 and a cathode electrode62 were formed.

The sialon phosphors having compositions of TABLE 1 were applied to thered phosphor, and the sialon phosphors were expressed by a compositionformula of the following equation (8).

(Sr_(1−x1)Eu_(x1))_(a)Si_(b)AlO_(c)N_(d)  (8)

The sialon phosphors having compositions of TABLE 2 were applied to thegreen phosphor, and the sialon phosphors were expressed by a compositionformula of the following equation (9).

(Sr_(1−x2)Eu_(x2))_(3−y)Si_(13−z)Al_(3+z)O_(2+u)N_(21−w)  (9)

The diameter r outside the red fluorescent layer was set to 105 μm, andthe diameter D outside the intermediate layer was set to 410 μm. Thediameter r and the diameter D satisfy the equation (2).

2.0r=210 (μm)≦D=410 (μm)≦r+1000=1105 (Ξm)

The white-color light emitting module was driven at 20 mA, and theluminous efficiency was evaluated by the total luminous flux using theintegrating sphere. TABLE 3 illustrates the result.

TABLE 1 Peak wavelength x1 a b c d (nm) Example 1 0.10 0.858 3.34 0.3504.92 622 Example 2 0.11 0.935 3.41 0.418 5.18 631 Example 3 0.15 0.9113.70 0.272 5.63 642 Example 4 0.08 0.680 2.54 0.332 4.29 616 Example 50.09 0.680 2.54 0.332 4.29 616

TABLE 2 Peak wavelength x2 y z u w (nm) Example 1 0.10 −0.08 0.11 −0.041.43 524 Example 2 0.08 −0.06 0.13 0.22 0.06 518 Example 3 0.10 −0.08−0.03 −0.06 0.09 520 Example 4 0.07 −0.07 −0.23 −0.03 0.79 511 Example 50.08 −0.06 −0.15 0.11 −0.11 516

Comparative Examples 1 to 5

The white-color light emitting modules were produced and evaluatedsimilarly to Examples 1 to 5 except that the green fluorescent layer wasdirectly formed on the red fluorescent layer while the intermediatelayer was not formed. TABLE 3 illustrates the result.

Comparative Examples 6 to 10

The white-color light emitting modules were produced and evaluatedsimilarly to Examples 1 to 5 except that the diameter D outside theintermediate layer was set to 2150 μm. TABLE 3 illustrates the result.The diameter r and the diameter D do not satisfy the equation (2).

2.0r=210 (μm)≦D=2150 (μm)≧r+1000=1105 (μm)

TABLE 3 Luminous efficiency Examples Comparative examples Luminousefficiency (lm/W) Examples 1-5 52~54 Comparative 48~50 examples 1-5Comparative 46~48 examples 6-10

As is clear from TABLE 3, it is confirmed that the luminous efficiencyis improved in Examples 1 to 5.

Examples 6 to 10

The white-color light emitting modules were produced and evaluatedsimilarly to Examples 1 to 5 except that an Eu activated alkaline earthorthosilicate phosphor was used as the green phosphor, the diameter routside the red fluorescent layer was set to 120 μm, and the diameter Doutside the intermediate layer was set to 505 Ξm. TABLE 4 illustratesthe result. The diameter r and the diameter D satisfy the equation (2).

2.0r=240 (μm)≦D=505 (μm)≦r+1000=1120 (μm)

Comparative Examples 11 to 15

The white-color light emitting modules were produced and evaluatedsimilarly to Examples 6 to 10 except that the green fluorescent layerwas directly formed on the red fluorescent layer while the intermediatelayer was not formed. TABLE 4 illustrates the result.

Comparative Examples 16 to 20

The white-color light emitting modules were produced and evaluatedsimilarly to Examples 6 to 10 except that the diameter D outside theintermediate layer was set to 2150 μm. TABLE 4 illustrates the result.The diameter r and the diameter D do not satisfy the equation (2).

2.0r=240 (μm)≦D=2150 (μm)≦r+1000=1120 (μm)

TABLE 4 Luminous efficiency (lm/W) Examples 6-10 54~56 Comparative 51~53examples 11-15 Comparative 48~50 examples 16-20

As is clear from TABLE 4, it is confirmed that the luminous efficiencyis improved in Examples 6 to 10.

Examples 11 to 15

The light emitting devices 20 of the second embodiment illustrated inFIG. 5, in which the near-ultraviolet LEDs were used as the lightemitting element, were connected so as to become a 4-by-4 arrayillustrated in FIG. 6, and the anode electrode 60 and the cathodeelectrode 62 were formed.

The sialon phosphors having compositions of TABLE 1 were applied to thered phosphor, and the sialon phosphors were expressed by the compositionformula of the following equation (8).

(Sr_(1−x1)Eu_(x1))_(a)Si_(b)AlO_(c)N_(d)  (8)

The sialon phosphors having compositions of TABLE 2 were applied to thegreen phosphor, and the sialon phosphors were expressed by thecomposition formula of the following equation (9).

(Sr_(1−x2)Eu_(x2))_(3−y)Si_(13−z)Al_(3+z)O_(2+u)N_(21−w)  (9)

(Sr,Ca,Ba)₁₀(PO₄)₆Cl₂:Eu²⁺ was used as the blue phosphor.

The diameter r outside the red fluorescent layer was set to 95 μm, andthe diameter D outside the intermediate layer was set to 320 μm. Thediameter r and the diameter D satisfy the equation (2).

2.0r=190 (μm)≦D=320 (μm)≦r+1000=1095 (μm)

Similarly to Examples 1 to 5, the white-color light emitting module wasdriven at 20 mA, and the luminous efficiency was evaluated by the totalluminous flux using the integrating sphere. TABLE 5 illustrates theresult.

Comparative Examples 21 to 25

The white-color light emitting modules were produced and evaluatedsimilarly to Examples 11 to 15 except that the green fluorescent layerwas directly formed on the red fluorescent layer while the intermediatelayer was not formed. TABLE 5 illustrates the result.

Comparative Examples 26 to 30

The white-color light emitting modules were produced and evaluatedsimilarly to Examples 11 to 15 except that the diameter D outside theintermediate layer was set to 1850 μm. TABLE 5 illustrates the result.The diameter r and the diameter D do not satisfy the equation (2).

2.0r=190 (Ξm)≦D=1850 (μm)≧r+1000=1095 (μm)

TABLE 5 Luminous efficiency (lm/W) Examples 11-15 47~49 Comparative42~44 examples 21-25 Comparative 39~41 examples 26-30

As is clear from TABLE 5, it is confirmed that the luminous efficiencyis improved in Examples 11 to 15.

Examples 16 to 20

The white-color light emitting modules were produced and evaluatedsimilarly to Examples 11 to 15 except that the Eu activated alkalineearth orthosilicate phosphor was used as the green phosphor, thediameter r outside the red fluorescent layer was set to 130 Ξm, and thediameter D outside the intermediate layer was set to 450 μm. TABLE 6illustrates the result. The diameter r and the diameter D satisfy theequation (2).

2.0r=260 (μm)≦D=450 (μm)≦r+1000=1130 (μm)

Comparative Examples 31 to 35

The white-color light emitting modules were produced and evaluatedsimilarly to Examples 16 to 20 except that the green fluorescent layerwas directly formed on the red fluorescent layer while the intermediatelayer was not formed. TABLE 6 illustrates the result.

Comparative Examples 36 to 40

The white-color light emitting modules were produced and evaluatedsimilarly to Examples 16 to 20 except that the diameter D outside theintermediate layer was set to 2150 μm. TABLE 6 illustrates the result.The diameter r and the diameter D do not satisfy the equation (2).

2.0r=260 (μm)≦D=2150 (μm)≧r+1000=1130 (μm)

TABLE 6 Luminous efficiency (lm/W) Examples 16-20 47~49 Comparative41~43 examples 31-35 Comparative 37~39 examples 36-40

As is clear from TABLE 6, it is confirmed that the luminous efficiencyis improved in Examples 16 to 20.

What is claimed is:
 1. A light emitting device comprising: a board; alight emitting element mounted on a principal surface of the board, thelight emitting element emitting light having a wavelength of 250 nm to500 nm; a red fluorescent layer formed on the light emitting element,the red fluorescent layer including a red phosphor expressed by equation(1), an outer circumference of the red fluorescent layer having asemicircular shape with a diameter r in a section perpendicular to theprincipal surface;(M_(1−x1)Eu_(x1))_(a)Si_(b)AlO_(c)N_(d)  (1) (In the equation (1), M isan element that is selected from IA group elements, IIA group elements,IIIA group elements, IIIB group elements except Al (Aliminum),rare-earth elements, and IVB group elements, and x1, a, b, c, and dsatisfy the following relationship:0<x1≦1,0.60<a<0.95,2.0<b<3.9,0.04≦c≦0.6,4<d<5.7) an intermediate layer formed on the red fluorescent layer, theintermediate layer being made of transparent resin, an outercircumference of the intermediate layer having a semicircular shape witha diameter D in a section perpendicular to the principal surface; and agreen fluorescent layer formed on the intermediate layer, the greenfluorescent layer including a green phosphor, an outer circumference ofthe green fluorescent layer having a semicircular shape in a sectionperpendicular to the principal surface, wherein a relationship betweenthe diameter r and the diameter D satisfies equation (2):2.0r (μm)≦d≦(r+1000) (μm).  (2)
 2. The device according to claim 1,wherein the green phosphor has a composition expressed by equation (3):(M′_(1−x2)Eu_(x2))_(3−y)Si_(13−z)Al_(3+z)O_(2+u)N_(21−w)  (3) (In theequation (3), M′ is an element that is selected from IA group elements,IIA group elements, IIIA group elements, IIIB group elements except Al(Aliminum), rare-earth elements, and IVB group elements, and x2, y, z,u, and w satisfy the following relationship:0<x2<1,−0.1≦y≦0.15,−1≦z≦1,−1<u−w≦1.5).
 3. The device according to claim 1, wherein the greenphosphor is an Eu activated alkaline earth orthosilicate phosphor. 4.The device according to claim 1, wherein the light emitting element is ablue LED.
 5. The device according to claim 1, wherein the transparentresin is a silicone resin.
 6. The device according to claim 1, whereinthe element M is a strontium (Sr).
 7. The device according to claim 2,wherein the element M′ is a strontium (Sr).
 8. The device according toclaim 1, further comprising an outer surface layer made of a transparentresin, the outer surface layer being formed on the green fluorescentlayer.
 9. The device according to claim 1, further comprising a yellowfluorescent layer including a yellow phosphor, the yellow fluorescentlayer being formed between the red fluorescent layer and theintermediate layer.